It is now known that kidney stones form from a variety of metabolic, environmental and physicochemical disturbances. Metabolic factors include hypercalciuria (high urinary calcium), hyperoxaluria (high urinary oxalate), hyperuricosuria (high urinary uric acid), hypocitraturia (low urinary citrate), and unusually acid or alkaline urinary medium (Pak, Cecil Textbook of Medicine, 17th Edition, pp. 628-633, 1985). Important environmental factors are low urine volume, high urinary sodium, high urinary sulfate, high urinary phosphate, and low urinary magnesium (Pak et al. J. Urology, October, 1985, in press). Physicochemical factors indicate whether urine samples are supersaturated (therefore likely to support stone formation) with respect to stone-forming salts (calcium oxalate, brushite, sodium urate, struvite and uric acid).
It is presumed that metabolic factors are due to disturbed metabolism of patients suffering from stones, whereas environmental factors originate from dietary aberrations. Physicochemical factors are dependent upon metabolic and environmental factors, since urinary saturation is calculated from various urinary excretions.
Although the measurement of above stone-forming factors/risks would be very helpful in making the appropriate diagonsis and in offering proper treatment, it has been hampered by the difficulty in adequately preserving and collecting urine samples and by the lack of an easily understood way of displaying results.
Thus, there is considerable uncertainty regarding exact methods for preserving urine which would not influence various analyses. It has been suggested that urine samples need to be acidified in order to prevent precipitation of calcium and magnesium salts, and allow proper determination of calcium, magnesium, phosphate and oxalate. Unfortunately, urinary acidification does not allow measurement of pH and uric acid, the latter due to its precipitation in an acid environment. Moreover, considerable care must be taken in adding acidic solution to urine, a task not enjoyed by patients. Freezing may prevent bacterial contamination and may preserve some urinary components (such as citrate). However, precipitation of uric acid and calcium salts may occur when samples are frozen; such precipitates are often hard to redissolve upon thawing. When samples are kept at 20.degree. C. or higher, there may be an incomplete recovery of citrate, pyrophosphate, uric acid and creatinine (due to bacterial contamination and ensuing bacterial enzymatic degradation). In contrast, oxalate content may well increase because it would be formed from ascorbic acid normally contained in urine. Thus, non-refrigerated urine samples may yield lower values for oxalate. The addition of appropriate antibiotics or disinfectants to control bacterial contamination may be helpful, but is not likely to prevent conversion of vitamin C to oxalate or precipitation of stone-forming salts. If urine samples are refrigerated during collection until the time of analysis (within three days of analysis), the urinary composition is maintained, except in few samples which are infected or overly concentrated. However, this procedure is impractical except in the setting of a research center.
Another problem in submitting urine samples for analysis of stone-forming risks to the laboratory is the need to measure total volume (usually 24-hour) accurately. Since shipping 24-hour collection is impractical, it would be advantageous to have total volume of urine measured so that small aliquot(s) of whole urine collection could be sent to the laboratory. Unfortunately, few patients have means to accurately measure urine volume.
Thus, a method of urine collection and preservation, which does not require refrigeration or determination of total volume, permitting accurate analysis of all stone-forming risks previously enumerated from aliquots of urine submitted, is clearly needed.
Even when metabolic and environmental factors could be accurately measured from proper urine collection and preservation system, such information would have a limited value to the physician taking care of the patient unless comparison with normal control value is provided, physicochemical factors are derived, and unless that information is displayed in a readily understandable fashion.